After the many requests for us warlocks to get green fire, Blizzard confirmed today that those requests and wishes did not fall on deaf ears:

Fel Flame (level 81): Quick-hitting spell dealing Shadowfire damage. This is similar to the mage ability Frostfire Bolt, in that the lower of the two resistances (in this case shadow and fire) on your target will be used for calculating its damage. Additionally, Fel Flame refreshes the duration of Immolate and Unstable Affliction. Our goal for Fel Flame is to provide a spell that's good for mobility and for use by Destruction and Demonology specs. Also, did we mention it uses green fire? Yep. Instant cast.

Now the question still remains whether or not green fire will replace all of our other spells, but I would assume they will change since it would be a little wierd to have one green fire spell and several other orange ones.